1. Technical Field
The invention is directed to an energy sensitive material and a process for device fabrication in which the energy sensitive resist material is used.
2. Art Background
Devices such as integrated circuits are complex structures made of a variety of materials. These materials are precisely configured to form the desired device by a variety of processes. A lithographic process is frequently used to transfer the desired configuration into a substrate to fabricate such devices.
Lithographic processes use intermediate materials frequently referred to as resists. A positive or negative image of the desired configuration is first introduced into the resist by exposing it to patterned radiation which induces a chemical change in the exposed potions of the resist. This chemical change is then exploited to develop a pattern in the resist, which is then transferred into the substrate underlying the resist.
The efficacy of a lithographic process depends at least in part on the resist used to transfer the pattern into the substrate. Certain types of resists offer particular advantages in the context of specific lithographic processes. For example, certain resists use chemical amplification to provide increased radiation sensitivity; i.e., the radiation produces a catalytic agent which induces a chemical event which propagates, resulting in multiple other chemical events. The effect of the radiation is amplified because one photon generates many chemical events.
Certain chemically amplified: resists contain a polymer in which the aqueous base solubility of the polymer is reduced by pendant protective groups which are removed from the polymer by the catalytic agent that is produced by incident radiation. The resist material in the exposed region, but not the unexposed region, consequently undergoes an increase in aqueous base solubility. The difference in the aqueous base solubility between the unexposed and exposed regions of the resist is then exploited to develop a topographic pattern.
One class of chemically amplified resists, disclosed in U.S. Pat. No. 4,939,070 to Ito et al., contains hydroxystyrene polymers that have recurrent pendant groups such as t-butoxycarbonyl (tBOC) groups. These groups replace the hydrogen in the hydroxyl moiety pendant to the styrene polymers phenyl moieties and reduce the inherent aqueous base solubility of the hydroxystyrene when so attached. These protective, masking groups are cleaved from the polymer, generally by heating after the exposure, in the presence of acid which is generated from a dissolved photoacid generator (PAG) by exposing the resist to radiation. When the masking groups are cleaved from such polymers, they are replaced by hydrogen atoms to yield free phenols and, as a result, the polymers aqueous base solubility changes from relatively insoluble to relatively soluble.
It is an objective of a lithographic process for device fabrication to transfer a device pattern from a resist into a substrate as accurately as possible. It follows that it is desirable to avoid shrinkage in patterned resists that cause a concomitant distortion in the resist image and the resulting device pattern. Because of the undesirable shrinkage that results when the masking groups are cleaved from the polymer, it is desirable to minimize the number of these groups that are cleaved to obtain the desired effect. One solution to the problem of polymer shrinkage during deprotection is described in U.S. Pat. No. 5,385,809, to Bohrer et al. In Bohrer et al. the polymers are partially deprotected prior to irradiation, thereby reducing the amount by which the polymer shrinks when exposed to radiation and thermally developed.
A resist shrinks when thee bulky protective groups are removed from the polymer in substantial numbers. The fewer the number of these protective groups that are removed per unit of polymer, the less the polymer will shrink. To obtain a pattern that is truer to the desired pattern, a chemically amplified resist that contains a smaller fraction of protective groups and shrinks less upon aleprotection is desired.